1. Field of the Invention
The present invention relates to a novel method of preparing polyimide prepolymers having excellent heat resistance from polyamines and dicarboxylic acids having aliphatic double bonds or derivatives thereof.
2. Description of the Prior Art
One of the methods well known for preparing polyimide prepolymers is a two stage process in which a double bond-containing imide compound is isolated after being prepared from a polyamine and an aliphatic dicarboxylic anhydride or a derivative thereof equivalent to the amino group of the polyamine and subsequently the obtained imide compound is reacted with a polyamine to prepare a polyimide prepolymer. For example, maleamic acid obtained by reaction of 1 mole of maleic anhydride with 0.5 mole of diaminodiphenylmethane undergoes imide ring closure to form bismaleimide (U.S. Pat. No. 3,839,358 for example) and the bismaleimide is reacted with diaminodiphenylmethane to yield a polyaminobismaleimide prepolymer. This method is conventionally used (U.S. Pat. No. 3,562,223 and U.S. Pat. No. 3,658,764 for example) and commercialized already.
However, this two stage method is economically disadvantageous because of the complexity of its first stage process in which a double bond-containing imide compound is prepared in advance, followed by isolation and refining. Therefore, there have been studied in the industry concerned one stage methods in which a polyamidic acid prepolymer obtained by reaction of dicarboxylic anhydride having aliphatic double bonds with a diamine of excess equivalent is subjected to dehydrating ring closure to prepare a polyimide prepolymer. As a result, two methods using different means of dehydrating ring closure are proposed. One of the two methods is the heat ring closure method in which an azeotropic water removing agent such as benzene (Published Unexamined Japanese patent application No. 4644/71 and 99296/73, for example) or a sulfuric acid catalyst (Published Unexamined Japanese patent application No. 3846/71) is usually used in order to accelerate the heat ring closure. The other one is the acetic anhydride method (Published Unexamined Japanese patent application No. 18119/74, for example) in which an acetic anhydride is used in combination with a metal salt of acetic acid and tertiary amine as the accelerator. However, the former heat ring closure method of these conventional one stage methods is almost impracticable because it has such drawbacks that the imide ring closure rate of the finally obtained polyimide prepolymer is as low as 50 to 70% to leave a considerable amount of amide unit and therefore that foaming occurs during curing of the prepolymer to form a number of voids in the molding. Also, the latter acetic anhydride method has the vital drawback that although the imide ring closure rate of the finally obtained polyimide prepolymer can be brought to a practicable level as high as more than 90%, the curing speed of the prepolymer during molding is very slow, and yet the heat resistance of the finally obtained cured molding is extremely low.